LIMIT AND FITS

LIMTS - These are two extreme permissible sizes of dimension between which actual size of dimension is contained .The greater of these two is called high limit and the smaller low limit.

FITS - It is the relationship existing between two mating parts with respect to amount of play or interference which is present when they are assembled together.  It is the degree of tightness or looseness between two mating parts to perform a definite function.

TERMINOLOGY-

Zero Line- It is a line along which represents the basic size and zero for measurement of upper or lower deviations.

Basic Size- It is the size with reference to which upper or lower limits of size are defined.

Shaft and Hole- These terms are used to designate all the external and internal features of any shape and not necessarily cylindrical.

Hole Designation - By upper case letters from A, B, … Z, 

Shaft Designation- By lower case letters from a, b, … z, - 25

BASIC DEFINITIONS-

Upper Deviation: The algebraic difference between the maximum limit of and the corresponding basic size.

Lower Deviation: The algebraic difference between the minimum limit of size basic size.

Fundamental Deviation:  It is one of the two deviations which is chosen to define the position of the tolerance zone.  Or nearest from both upper and lower deviations.

Tolerance: The algebraic difference between upper and lower deviations. It is an absolute value.

Limits of Size: There are two permissible sizes for any particular dimension between which the actual size lies, maximum and minimum.

INTERNATIONAL TOLERANCE GRADES
The variation in part size, also called the magnitude of the tolerance zone, and is expressed in grade or IT numbers. Seven grade numbers are used for high-precision parts; these are example
ITOl, ITO, ITl, IT2, IT3, IT4, IT5
The most commonly used grade numbers are IT6 through IT16, and these are based on the Renard R5 geometric series of numbers .For these, the basic equation is calculation of  “i”